1. Field of the Invention
The present invention relates to a liquid crystal display apparatus comprising thin film transistors. More particularly, the invention relates to a liquid crystal display apparatus having a structure with electrodes which are faced with pixel electrodes and which generate cumulative capacity.
2. Description of the Related Art
FIGS. 10 and 11 show a typical constitution of a thin film transistor array substrate used by a conventional thin film transistor liquid crystal display apparatus. The substrate carries on its surface gate wires G and source wires S.
The thin film transistor array substrate of FIGS. 10 and 11 is a transparent substrate 13 illustratively made of glass having gate wires G and source wires S deposited thereon in matrix fashion. Each of the areas surrounded by the gate wires C and source wires S serves as one pixel 1. Each pixel is furnished with a thin film transistor T100.
The thin film transistors T100 shown in FIGS. 10 and 11 are of common, inversely staggered type. Each gate wire G and a gate electrode 2 forming part of the gate wire G in question are covered with a gate insulating film 3. On the gate insulating film 3 over the gate electrode 2 is a semiconductor active film 4 which is made of amorphous silicon (a-Si) and which faces the gate electrode 2. A drain electrode 6 and a source electrode 7, both made of a conductive material, are positioned face to face on two edges of the semiconductor active film 4. The upper portions of the two edges of the active film 4 are covered with ohmic contact films 8, 8 formed illustratively of amorphous silicon doped with impurities (i.e., donors) such as phosphorus of a high density.
A passivation film 10 made of an insulating film is deposited over the gate insulating film 3, source electrodes 6 and drain electrodes 7. On the passivation film 10 are pixel electrodes 11 which, constituted by a transparent conductive material such as ITO (indium tin oxide), cover almost all pixels 1 ranging from top to side of the drain electrodes 6. The pixel electrodes 11 and the passivation film 10 are covered with an oriented film, not shown. Above the oriented film are liquid crystal and an opposite substrate having common electrodes. The whole structure constitutes an active matrix liquid crystal display apparatus. When a transparent pixel electrode 11 applies an electric field to liquid crystal molecules, the orientation of the molecules is controlled as desired.
In the liquid crystal display apparatus having the constitution of FIGS. 10 and 11, auxiliary electrodes 12 formed simultaneously with the gate electrodes 2 on the substrate 13 are installed opposite to the pixel electrodes 11. As illustrated in FIG. 11, the auxiliary electrodes 12 are furnished so as to surround the contour of each pixel 1 corresponding to the circumference of each pixel electrode 11. Each pixel electrode 11 and its corresponding auxiliary electrode 12 sandwich the passivation film 10 to constitute a capacitor providing a cumulative capacity that is used to inhibit the adverse effects of a parasitic capacity generated naturally upon liquid crystal activation.
In the liquid crystal display apparatus of the above constitution, the transparent substrate 13 is usually backlighted. The backlight is shielded by, or allowed to transmit, the orientation-controlled liquid crystal to let the user recognize the contrast on display.
In the constitution shown in FIGS. 10 and 11, each pixel electrode 11 and its corresponding auxiliary electrode 12 sandwich the gate insulating film 3 and passivation film 10 to form the cumulative capacity. While advantageous in driving the liquid crystal, the above constitution is known to have the following major deficiencies:
The gate insulating film 3 is interposed between the gate electrodes 2 and the semiconductor active film 4 for insulation purposes. To attain the good insulating property of the gate insulating film 3 requires strictly managing conditions for forming the film. On the other hand, the passivation film 10 is generally formed under less severe conditions than the gate insulating film 3 because the quality demanded of the passivation film is not so high.
In the conventional structure of FIGS. 10 and 11 where each pixel electrode 11 and its corresponding auxiliary electrode 12 sandwich the gate insulating film 3 and passivation film 10 to form cumulative capacity, two insulating films constitute the cumulative capacity. This makes it difficult to secure appreciable quantities of cumulative capacity. It is also difficult to control the cumulative capacity level to a desired value.